Often, in an underground well, several casings placed concentrically extend down the well from the opening of the wellbore to define and protect drilling and production equipment from the surrounding formations and vice versa. The lengths of the casings decrease with increasing diameters, so that the casing having the smallest diameter extends the farthest down the well. The outermost casing is generally cemented into the formation over the entire length of the pipe, whereas the rest of the casings are generally only cemented in a lower portion of the length of the casing from a guide shoe upwards in the annulus. The annuli between the different casings are generally, at least in an upper portion, filled by old, settled-out drilling mud of large mud weight.
In some cases, it may be desirable to remove a length of the innermost casing. This may be, for example, in connection with establishing a new well path. The viscous and/or settled-out drilling mud bring about drag on the casing so large that it will only be possible to remove shorter lengths, maybe only 5-10 meters, in each lifting operation, and it will therefore be very time-consuming and expensive to remove larger lengths of the casing.